1. Field of the Invention
The present invention relates to a method of controlling a resistance welding machine used for spot welding, for example.
2. Prior Art
Resistance welding, such as spot welding, has been used for a variety of products formed of steel sheets. However, weld defects during resistance welding tend to increase these days. In other words, conventionally, workpieces were generally formed of mild steel sheets. Therefore, improper welding occurred less frequently. By controlling welding conditions constant, it was possible to maintain the quality of welding relatively stably. However, galvanized steel sheets and high-tensile steel sheets have begun to be used abundantly instead of mild steel sheets, and weld defects have occurred more frequently.
Accordingly, the advent of a method capable of accurately controlling the quality of welding has been waited for.
To cope with this problem, a variety of welding control methods have been developed. For example, one of the methods developed up to this time is a method wherein the resistance across the electrode tips is obtained from welding current and welding voltage, and the welding current is controlled on the basis of the change pattern of the resistance. An example of this type is disclosed in Japanese Laid-open Patent Application No. Sho 57-127584. Furthermore, another method has been developed wherein the voltage across the electrode tips is with a preset reference voltage changing with time, and welding control is carried out depending on whether the difference therebetween is within an allowable value or not. An example of this type is disclosed in Japanese Patent Publication No. Sho 59-40551. Moreover, in accordance with the progress of the recent computer and simulation technologies, other methods have been developed and used practically wherein a thermal conduction model is used, and nugget diameters are calculated by using a computer. In these methods, the temperature distribution of a base metal is calculated from a thermal conduction model, nugget formation conditions are estimated from the temperature distribution, and welding control is carried out depending on the conditions. An example of this type is disclosed in Japanese Laid-open Patent Application No. Hei 9-216072. In addition, still another method has been developed wherein the temperature distribution of the base metal is calculated from a thermal conduction model, the nugget diameter is estimated from the temperature distribution, and the temperature distribution is corrected by using the movement amount of the electrodes during welding. An example of this type is disclosed in Japanese Laid-open Patent Application No. Hei 7-16791.
Among these technologies, in the cases of the conventional various welding control methods not using any thermal conduction models, it is necessary to carry out preliminary experiments for each welding material at a welding site to obtain the relationship between the quality of welding and its criterion. The result of the control is unsatisfactory. This disadvantage in the conventional welding control methods is caused by the fact that the control algorithm thereof is created on the basis of only the basic images and experimental equations.
Furthermore, the recent welding control methods using a thermal conduction model have a possibility of solving the above-mentioned problems since general-purpose control methods are incorporated. However, the actual welding conditions at a welding site depend on the mixture of various sheet combinations (the combinations of sheets being different in thickness, material and surface treatment), the presence or absence of welded points, the presence or absence of end point welding (welding to a sheet end portion), and the difference in shape between the electrode tips, and the like. Therefore, in some cases, it has been difficult to raise the accuracy of control and to obtain high welding quality by using only the thermal conduction model.
For example, when a sheet combination is formed of thin and thick sheets, and when three or more sheets are overlaid and welded, even if it is estimated that melted portions are sufficiently obtained by using the thermal conduction model depending on the contact interface positions of thin and thick sheets, it is difficult to judge whether the portion of the thin sheet making contact with the welding electrode functioning as a cooling end has melted or not.
Accordingly, the present invention is intended to provide a resistance welding machine control method capable of improving control accuracy and welding quality.